Expand contributor documentation

- Move from CONTRIBUTING.md to a subdirectory in docs, so
  we can expand and add more documentation.
- Move from markdown to reStructuredTest
- Add a direct blurb in the JupyterHub docs index page on
  how contribution.
- More prominent link to the Code of Conduct
- Add section on getting in touch with the JupyterHub community

docs/source/contributing/community.rst

@@ -0,0 +1,25 @@
+.. _contributing/community:
+
+================================
+Community communication channels
+================================
+
+We use `Gitter <https://gitter.im>`_ for online, real-time text chat. The
+primary channel for JupyterHub is `jupyterhub/jupyterhub <https://gitter.im/jupyterhub/jupyterhub>`_.
+Remember that our community is distributed across the world in various
+timezones, so be patient if you do not get an answer immediately!
+
+GitHub issues are used for most long-form project discussions, bug reports
+and feature requests. Issues related to a specific authenticator or
+spawner should be directed to the appropriate repository for the
+authenticator or spawner. If you are using a specific JupyterHub
+distribution (such as `Zero to JupyterHub on Kubernetes <http://github.com/jupyterhub/zero-to-jupyterhub-k8s>`_
+or `The Littlest JupyterHub <http://github.com/jupyterhub/the-littlest-jupyterhub/>`_),
+you should open issues directly in their repository. If you can not
+find a repository to open your issue in, do not worry! Create it in the `main
+JupyterHub repository <https://github.com/jupyterhub/jupyterhub/>`_ and our
+community will help you figure it out.
+
+A `mailing list <https://groups.google.com/forum/#!forum/jupyter>`_ for all
+of Project Jupyter exists, along with one for `teaching with Jupyter
+<https://groups.google.com/forum/#!forum/jupyter-education>`_.